1. Field of the Invention
The present invention pertains to a method for the regeneration of a particle filter arranged in the exhaust gas tract of an internal combustion engine and to a device for the regeneration of a particle filter arranged in the exhaust gas tract of an internal combustion engine. The invention pertains in particular to a method and to a device for regenerating particle filters in internal combustion engines operating with excess air such as diesel engines and gasoline engines with direct injection like those typically used in motor vehicles or commercial vehicles.
2. Description of the Related Art
To minimize the fine, carbon-containing particles, “particle separators” or particle filters are used in motor vehicles. A typical particle separator arrangement for motor vehicles is known from EP 10 727 65 A2. These particle separators differ from particle filters in that the exhaust gas stream is conducted along the separator structures, whereas, in the case of particle filters, the exhaust gas is forced to flow through the filter medium. As a result of this difference, particle filters tend to clog, which increases the exhaust gas backpressure. Particle filters cause an undesirable increase in the pressure at the exhaust gas outlet of the internal combustion engine, which in turn reduces engine power and leads to an increase in the amount of fuel consumed by the internal combustion engine. An example of a particle filter arrangement is known from EP 03 418 32 A2.
In both of the arrangements above, an oxidation catalyst located upstream of the particle separator or particle filter oxidizes nitrogen monoxide (NO) in the exhaust gas to nitrogen dioxide (NO2) with the help of the residual oxygen (O2) also present in the exhaust gas according to the following equation:2NO+O2<->2NO2.
At high temperatures, the equilibrium of the above reaction is on the NO side. This means that, as a result of this thermodynamic limitation, the NO2 levels that can be achieved at high temperatures are limited.
In the particle filter, the NO2 reacts with the extremely fine carbon-containing particles to form CO, CO2, N2, and NO. The strong oxidizing agent NO2, has the effect of continuously removing the deposited ultrafine particles, so that the complicated regeneration cycles which must be conducted in the case of other arrangements can be omitted. We speak in this context of a “passive” regeneration according to the following equations:2NO2+C->2NO+CO2 NO2+C->NO+CO2C+2NO2->N2+2CO2 
In addition to NO2, SO3 is also formed, the latter being produced on the platinum-containing NO oxidation catalysts from the sulfur contained in the fuel and/or motor oil. The SO3 and NO2 condense on cold spots in the exhaust gas tract and form highly corrosive sulfuric acid and nitric acid. Because of the sulfuric and nitric acids, the exhaust gas system must be made of high-grade steel up as far as the particle filters to avoid corrosion.
If all of the carbon deposited in the particle filter cannot be oxidized with the NO2, the carbon content and thus the exhaust gas backpressure increases continuously. To avoid this, more and more particle filters are currently being provided with a catalytic coating for the oxidation of NO as shown in EP 03 418 32 A2. In concrete terms, these are platinum-containing catalysts. The disadvantage of this method is that the NO2 formed on the particle filter can serve to oxidize only the particles which have been separated downstream of the layer catalytically active for NO oxidation, that is, only particles inside the filter medium. If, however, a layer of separated particles—a so-called “filter cake”—forms on the surface of the filter and thus on the catalytically active layer, the NO oxidation catalyst on the particle filter side then lies downstream of the filter cake, so that the soot particles separated cannot be oxidized with the help of NO2 from the NO oxidation catalyst applied to the particle filter. To be precise, only the catalyst layer applied on the raw gas side contributes to the performance of the system, because the NO2 formed catalytically on the clean gas side can no longer come into contact with the soot on the raw gas side or the soot deposited inside the filter material.
Another problem with particle filter coatings is that the geometric surfaces of the filter are much smaller than those of the catalyst substrates normally used. The reason for this is that the filters require relatively large free cross sections and thus free volumes on the raw gas side to allow the deposition of soot and motor oil ash. If ceramic filter substrates are used, this is realized by a low cell density of 50 cells per square inch (cpsi) to 200 cpsi. In contrast, pure catalysts are usually made with cell densities of 400-900 cpsi. Increasing the density from 50 cpsi to 900 cpsi results in an increase in the geometric surface area from 1 m2/L to 4 m2/L, as a result of which it becomes possible to achieve a considerable increase in the conversions on the catalysts.
For these reasons, even if the filter has been provided with a catalytic coating, it is impossible to dispense with an NO oxidation catalyst upstream of the particle filter, which means that the filter unit becomes relatively bulky. This situation exists even in cases where the NO oxidation catalysts and the particle filters form a single structural unit, in which the inlet area of the particle filter is designed as an NO oxidation catalyst as described, for example, in DE 103 270 30 A1.
Although it is possible through these measures to oxidize soot at temperatures as low as about 250° C., there are nevertheless applications in which exhaust gas temperatures do not reach 250° C. and thus the reliable function of the particle filter can no longer be guaranteed. This normally occurs when engines are operating at low loads and in engines installed in motor vehicles such as passenger cars, route busses, or garbage trucks which comprise long periods of no-load operation. In cases such as these, a second particle filter regeneration is used, in which the exhaust gas temperature is actively increased. This is usually done by the addition of hydrocarbons (HCs) upstream of the catalysts, especially HC oxidation catalysts. As a result of this exothermic reaction or oxidation of the hydrocarbons on the catalysts, a significant increase in temperature is achieved.
If it is possible in this way to increase the temperature beyond 600° C., the carbon deposited in the particle filter is oxidized or burned off with the help of oxygen according to the following equation:C+O2->CO2.
Nevertheless, there is the danger with this so-called “active” filter regeneration that, as a result of the exothermic burning-off of the carbon-containing soot, the temperature can increase sharply to as much as 1,000° C., and thus in most cases there will be damage to the particle filter and/or to the downstream catalysts. Because, furthermore, the increase in temperature must be maintained for several minutes to ensure quantitative oxidation of the soot particles, the amount of hydrocarbons required is not inconsiderable. This lowers the efficiency of the internal combustion engine, because the fuel for the engine is normally used as the source of the hydrocarbons.
A simple combination of these two types of regeneration, according to which hydrocarbons are added upstream of NO oxidation catalysts, does not lead to the goal either.
As a result of the increase in temperature to more than 600° C., hardly any more NO2 is formed on the NO oxidation catalysts because of the thermodynamic limitation. The oxidation of NO is also hindered by the large quantities of hydrocarbons, which results in a considerable decrease in the formation of NO2. This has the result that the particles must be oxidized with the help of oxygen alone, because there is no NO2 available in this phase. This prolongs the regeneration time.
At the same time, the NO oxidation catalysts are more vulnerable to thermal damage than catalysts for hydrocarbon oxidation, because, at temperatures over 550° C., the active components are subject to irreversible sintering, which reduces the NO oxidation activity.